Project Summary The long-term goal of this project is to determine the extent, regulatory mechanisms, and functional consequences of regulated pre-mRNA processing in skeletal muscle. A large number of genes express pre- mRNAs that undergo alternative splicing to produce conserved muscle-specific protein isoforms the functions of which are unknown. These isoforms often appear during late fetal or early postnatal development. Disruption of alternative splicing is a common feature of diseases affecting skeletal muscle, often involving reversion to fetal isoforms, yet little is known about the contributions of these changes to pathogenesis. An underlying hypothesis of this proposal is that a focus on genes with conserved fetal and adult protein isoforms, including adult isoforms that are muscle-specific, will discover not only previously unknown isoform-specific functions but also previously unknown gene functions critical for adult muscle homeostasis. To identify the functions of alternative splicing events in vivo we are using CRISPR-mediated removal of exons that undergo muscle- specific and/or postnatally regulated inclusion. In the first aim, we will determine the functions of a striated muscle-specific isoform of the Map4 microtubule-binding protein in which deletion of the muscle-specific exon significantly disrupts microtubule architecture in skeletal muscle myofibers and impacts muscle force generation. In the second aim we will determine the function of the skeletal muscle-specific Limch1 isoform, the absence of which also decreases skeletal muscle function in vivo. In the third aim we will use CRISPR- mediated introduction of epitope tags into endogenous genes to monitor the temporal and spatial details of the alternative splicing transitions and adult muscle-specific isoforms at the level of individual cells in vivo. This study is expected to identify previously unknown gene functions, increase understanding of adult skeletal muscle homeostasis and the impact of its disruption in disease.